<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html><head>
<title>SQLite Query Language: VACUUM</title>
<style type="text/css">
body {
    margin: auto;
    font-family: Verdana, sans-serif;
    padding: 8px 1%;
}

a { color: #044a64 }
a:visited { color: #734559 }

.logo { position:absolute; margin:3px; }
.tagline {
  float:right;
  text-align:right;
  font-style:italic;
  width:300px;
  margin:12px;
  margin-top:58px;
}

.toolbar {
  text-align: center;
  line-height: 1.6em;
  margin: 0;
  padding: 0px 8px;
}
.toolbar a { color: white; text-decoration: none; padding: 6px 12px; }
.toolbar a:visited { color: white; }
.toolbar a:hover { color: #044a64; background: white; }

.content    { margin: 5%; }
.content dt { font-weight:bold; }
.content dd { margin-bottom: 25px; margin-left:20%; }
.content ul { padding:0px; padding-left: 15px; margin:0px; }

/* rounded corners */
.se  { background: url(images/se.gif) 100% 100% no-repeat #044a64}
.sw  { background: url(images/sw.gif) 0% 100% no-repeat }
.ne  { background: url(images/ne.gif) 100% 0% no-repeat }
.nw  { background: url(images/nw.gif) 0% 0% no-repeat }

/* Things for "fancyformat" documents start here. */
.fancy img+p {font-style:italic}
.fancy .codeblock i { color: darkblue; }
.fancy h1,.fancy h2,.fancy h3,.fancy h4 {font-weight:normal;color:#044a64}
.fancy h2 { margin-left: 10px }
.fancy h3 { margin-left: 20px }
.fancy h4 { margin-left: 30px }
.fancy th {white-space:nowrap;text-align:left;border-bottom:solid 1px #444}
.fancy th, .fancy td {padding: 0.2em 1ex; vertical-align:top}
.fancy #toc a        { color: darkblue ; text-decoration: none }
.fancy .todo         { color: #AA3333 ; font-style : italic }
.fancy .todo:before  { content: 'TODO:' }
.fancy p.todo        { border: solid #AA3333 1px; padding: 1ex }
.fancy img { display:block; }
.fancy :link:hover, .fancy :visited:hover { background: wheat }
.fancy p,.fancy ul,.fancy ol { margin: 1em 5ex }
.fancy li p { margin: 1em 0 }
/* End of "fancyformat" specific rules. */

</style>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
  
</head>
<body>
<div><!-- container div to satisfy validator -->

<a href="index.html">
<img class="logo" src="images/sqlite370_banner.gif" alt="SQLite Logo"
 border="0"></a>
<div><!-- IE hack to prevent disappearing logo--></div>
<div class="tagline">Small. Fast. Reliable.<br>Choose any three.</div>

<table width=100% style="clear:both"><tr><td>
  <div class="se"><div class="sw"><div class="ne"><div class="nw">
  <table width=100% style="padding:0;margin:0;cell-spacing:0"><tr>
  <td width=100%>
  <div class="toolbar">
    <a href="about.html">About</a>
    <a href="sitemap.html">Sitemap</a>
    <a href="docs.html">Documentation</a>
    <a href="download.html">Download</a>
    <a href="copyright.html">License</a>
    <a href="news.html">News</a>
    <a href="support.html">Support</a>
  </div>
<script>
  gMsg = "Search SQLite Docs..."
  function entersearch() {
    var q = document.getElementById("q");
    if( q.value == gMsg ) { q.value = "" }
    q.style.color = "black"
    q.style.fontStyle = "normal"
  }
  function leavesearch() {
    var q = document.getElementById("q");
    if( q.value == "" ) { 
      q.value = gMsg
      q.style.color = "#044a64"
      q.style.fontStyle = "italic"
    }
  }
</script>
<td>
    <div style="padding:0 1em 0px 0;white-space:nowrap">
    <form name=f method="GET" action="http://www.sqlite.org/search">
      <input id=q name=q type=text
       onfocus="entersearch()" onblur="leavesearch()" style="width:24ex;padding:1px 1ex; border:solid white 1px; font-size:0.9em ; font-style:italic;color:#044a64;" value="Search SQLite Docs...">
      <input type=submit value="Go" style="border:solid white 1px;background-color:#044a64;color:white;font-size:0.9em;padding:0 1ex">
    </form>
    </div>
  </table>
</div></div></div></div>
</td></tr></table>
<div class=startsearch></div>
  
<h1 align="center">SQL As Understood By SQLite</h1><p><a href="lang.html">[Top]</a></p><h2>VACUUM</h2><h4><a href="syntaxdiagrams.html#vacuum-stmt">vacuum-stmt:</a></h4><blockquote> <img alt="syntax diagram vacuum-stmt" src="images/syntax/vacuum-stmt.gif"></img> </blockquote>


<p>
  The VACUUM command rebuilds the entire database. There are several 
  reasons an application might do this:

<ul>
  <li> <p> Unless SQLite is running in "auto_vacuum=FULL" mode, when a large
     amount of data is deleted from the database file it leaves behind empty
     space, or "free" database pages. This means the database file might
     be larger than strictly necessary. Running VACUUM to rebuild the 
     database reclaims this space and reduces the size of the database file.

  <li> <p> Frequent inserts, updates, and deletes can cause the database file
     to become fragmented - where data for a single table or index is scattered 
     around the database file. Running VACUUM ensures that each table and
     index is largely stored contiguously within the database file. In some
     cases, VACUUM may also reduce the number of partially filled pages in
     the database, reducing the size of the database file further.

  <li> <p> Normally, the database <a href="pragma.html#pragma_page_size">page_size</a> and whether or not the database
     supports <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> must be configured before the database file is
     actually created. However, when not in <a href="wal.html">write-ahead log</a> mode, the 
     <a href="pragma.html#pragma_page_size">page_size</a> and/or <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> properties of an existing database may be
     changed by using the <a href="pragma.html#pragma_page_size">page_size</a>  and/or 
     <a href="pragma.html#pragma_auto_vacuum">pragma auto_vacuum</a> pragmas and then immediately VACUUMing
     the database. When in <a href="wal.html">write-ahead log</a> mode, only the <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a>
     support property can be changed using VACUUM.
</ul>

<p>VACUUM only works on the main database. It is not possible to VACUUM an
attached database file.

<p>The VACUUM command works by copying the contents of the database into
a temporary database file and then overwriting the original with the 
contents of the temporary file. When overwriting the original, a rollback
journal or <a href="wal.html">write-ahead log</a> WAL file is used just as it would be for any
other database transaction. This means that when VACUUMing a database, 
as much as twice the size of the original database file is required in free
disk space.

<p>The VACUUM command may change the <a href="lang_createtable.html#rowid">ROWIDs</a> of entries in any
tables that do not have an explicit <a href="lang_createtable.html#rowid">INTEGER PRIMARY KEY</a>.
</p>

<p>A VACUUM will fail if there is an open transaction, or if there are one or
more active SQL statements when it is run.

<p>As of SQLite version 3.1, an alternative to using the VACUUM command to
reclaim space after data has been deleted is auto-vacuum mode, enabled using
the <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> pragma. When <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> is enabled for a database
free pages may be reclaimed after deleting data, causing the file to shrink,
without rebuilding the entire database using VACUUM.  However, using
<a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> can lead to extra database file fragmentation.  And <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a>
does not compact partially filled pages of the database as VACUUM does.

</p>



